1. Field of the Invention
The present invention relates to the purification of extracted phosphoric acid, especially to purification involving removal of dissolved silica from the extracted phosphoric acid.
2. Description of the Prior Art
Wet process phosphoric acid prepared by treating a phosphate rock with a mineral acid such as sulfuric acid, hydrochloric acid, nitric acid, and the like, contains metallic component impurities such as aluminum, calcium, sodium, potassium, magnesium, chromium, titanium and the like and also impurities such as mineral acids such as sulfuric acid, fluorine, silica and organic materials. Various techniques have been proposed for purifying wet process phosphoric acid to enable its use for preparation of not only fertilizers but also industrial products, food additives and the like. Purification of wet process phosphoric acid has been conducted industrially.
As mentioned above, silica is dissolved in wet process phosphoric acid with a content usually in the range of from 0.2 to 1 wt%. Various methods of removing such silica from phosphoric acid are known. However, all of the known methods are unsatisfactory for decreasing the silica content to the degree required for use of phosphoric acid as a food additive. For example, a silicofluoride recovery method in which silica is precipitated and separated as siliconfluoride by adding a precipitating agent such as sodium hydroxide, sodium carbonate, sodium chloride, potassium hydroxide and the like is known. However, because of the solubility of the silicon-fluoride, the silica removal coefficient is only about 60-70 wt.% even though an excess of the precipitating agent is used.
Japanese unexamined patent publication No. 34495/1974 discloses a method of removing silica as tetrafluorosilicate by heating phosphoric acid containing silica and fluorine to a temperature higher than 100.degree. C. and injecting steam into the phosphoric acid. In this method, the phosphoric acid containing fluorine is heated to a high temperaure. Accordingly, corrosion of the apparatus used has been difficult to prevent and the amount of steam used must be increased because vaporization of the tetrafluorosilicate decreases in proportion to the decrease in the concentration of silica. Accordingly, the concentration of phosphoric acid is decreased at a ratio of about 5-20 wt% as P.sub.2 O.sub.5.
It has also been known that a wet process phosphoric acid can be extracted with an organic solvent which dissolves the phosphoric acid and has low solubility to water; alcohols, e.g., n-butanol, isamyl alcohol, isobutanol: ethers; e.g., isopropyl ether; ketones, e.g., methylethyl ketone; phosphoric acid esters, e.g., tributyl phosphate and amines and the like. Thereafter, phosphoric acid is extracted with water from the extracted solution of phosphoric acid. In accordance with this process, metallic component impurities are substantially removed. However, the dissolved silica usually remains in the extracted phosphoric acid in a ratio of 100-3000 wt ppm.
In the specification, the term "extracted phosphoric acid" refers to a phosphoric acid prepared by purification by the extraction method and also to a concentrated phosphoric acid thereof. When the extracted phosphoric acid is concentrated without any other treatment, silica is precipitated as a scale (SiO.sub.2.nH.sub.2 O) on the surface of the heating tubes of the vaporizer whereby the thermal efficiency is decreased or the tube becomes clogged. When the extracted phosphoric acid is stored, amorphous silica is precipitated causing turbidity and a scale adheres on the surface of the storage tank. When phosphoric acid salts are prepared by using such an extracted phosphoric acid, turbidity of the neutralized mother liquor is caused by the amorphous silica.
Moreover, the inventors have found that various difficulties are caused by the dissolved silica present in the extracted phosphoric acid and have attempted to overcome this trouble. As a result, the inventors have discovered that the dissolved silica is adsorbed by silica gel and that the adsorption coefficient is highly affected by the fluorine content. In order to decrease the amount of dissolved silica in the extracted phosphoric acid, the silica component can be removed from the wet process phosphoric acid. However, the inventors have found that it is superior to treat extracted phosphoric acid with silica gel in comparison with treatment of wet process phosphoric acid with silica gel.